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Can you help me track down a fuel problem?
smart51 - 17/7/23 at 09:36 PM

This one has me stumped. How do I find this?

I have a 1970s Fiat 500. The day after it has been used, there is a strong smell of petrol in the cabin, but no smell under the bonnet (where the fuel tank is), in the engine bay at the back or in the garage where it is parked. It takes ages to start if it has not been used very recently - the fuel pump is engine driven and takes time to fill the carburettor. Restarts are very quick though. The car runs well once started.

A modern rubber fuel hose connects to an outlet on the top of the fuel tank. It goes through the front bulkhead and runs down the transmission tunnel inside the car, then out of the open end of the tunnel at the rear under the car. It is connected to a short length of copper pipe that goes through a bulkhead into the engine bay. From there a rubber hose goes to the fuel pump and another from there to the carburettor. There is no return pipe.

I've had the carpet out and the inspection covers off the tunnel. The fuel hose seems in good condition and is dry. There are no signs of fuel leaks, though I may not have seen every millimetre of pipe by eye.

I assume fuel is weeping or evaporating from the hose somewhere on the inside of the car. What is a good way of finding where from?

Dingz - 17/7/23 at 10:13 PM

I suggest you fit a solid fuel pipe inside the car and only use the minimum amount of flexible hoses. It may sound odd but Volatile fluids/gasses will permeate through rubber type hoses. They can be constructed to minimise the rate of permeation as on air con hoses but a normal fuel hose won’t be.

coyoteboy - 17/7/23 at 11:24 PM

Ooof fuel run inside the cabin, no thanks, not a fan of immolation. Will only take the tiniest of weeps to stink.

adithorp - 18/7/23 at 06:32 AM

What clips are on the hose connections. Worldwide (jubilee type) aren't right on small diameter hoses as they don't change radius under the screw, form a D shape and leak in the corners.

Other than that hose leaking somewhere. It could be a tiny one that evaporates almost as quick as it leaks. You could try putting talc around the hose and see if it gets washed away anywhere.

adithorp - 18/7/23 at 07:29 AM

You say "...open end of the tunnel at the rear..."?
Could be a leak after that that runs down the pipe into the cabin. Maybe you only smell it enclosed in there and there's enough air movement for it to dissipate in the garage? Might only be a tiny weep.

Prof_Cook - 18/7/23 at 08:37 AM

Worked on something similar years ago, found this forum which discussed the same problem - seems someone there replaced rubber pipe with braided pipe and also had carb overflow problem: https://www.fiatforum.com/threads/petrol-smell-in-cabin.426215/

smart51 - 18/7/23 at 08:41 AM

quote:
Originally posted by adithorp
You say "...open end of the tunnel at the rear..."?
Could be a leak after that that runs down the pipe into the cabin. Maybe you only smell it enclosed in there and there's enough air movement for it to dissipate in the garage? Might only be a tiny weep.


It isn't a real transmission tunnel, it houses cables (throttle, clutch, choke and starter (yes!)), gear linkage, brake lines and heater duct. The flat floor has sheet metal spot welded to the inside to form the tunnel. But it is open at the rear for everything to come out.

smart51 - 18/7/23 at 08:51 AM

quote:
Originally posted by Prof_Cook
Worked on something similar years ago, found this forum which discussed the same problem - seems someone there replaced rubber pipe with braided pipe and also had carb overflow problem: https://www.fiatforum.com/threads/petrol-smell-in-cabin.426215/
Thanks for the link. Their search function didn't bring that up for me. I may be being over dramatic. The smell isn't strong, but it is there.

I'll check the carb too, to be sure. I have the heater flap closed and there's no trace of smell in the engine bay, but always good to check.

russbost - 18/7/23 at 12:23 PM

First thing I would do would be to replave the rubber in the seating area for copper, as already said, rubber can "weep" & create a smell even though not visibly leaking

smart51 - 18/7/23 at 12:49 PM

quote:
Originally posted by adithorp
What clips are on the hose connections. Worldwide (jubilee type) aren't right on small diameter hoses as they don't change radius under the screw, form a D shape and leak in the corners.


Most are proper fuel hose clips, one is a jubilee clip (which is done up very tight). One is missing which I'll replace.

I did a test, I started the engine (which took lot of cranking before it fired) and ran it for a few seconds then switched it off. I started the engine again and it fired right away. It does suggest all the fuel in the carb is evaporating away. I'm a bit ignorant of carburettors, is that normal?

I've been for a drive and inspected the pipes before and after. There is no evidence of any leaks at any of the joins. As a test, I've left the engine bay lid open. Any fumes from there should dissipate rather than build up in the cabin.

[Edited on 18-7-2023 by smart51]

rf900rush - 19/7/23 at 06:43 AM

Don't forget E10 fuel is not good for older rubber and platics.
I have a 1970's speed boat that had a fibreglass tank which was damaged in weeks of using E10 petrol.

If the fuel lines or carbs use plastic or rubber this may also cause issues.

SteveWalker - 19/7/23 at 10:19 AM

I don't know the 500, but IIRC, its replacement, the 126, used the tunnel as an air duct, taking air from the engine air-cooling fan to the cabin vents and had a simple, hand-operated damper (just in front of the rear seat) to close it off. If the 500 is similar, does closing that off just before stopping make any difference?

smart51 - 19/7/23 at 10:27 AM

quote:
Originally posted by SteveWalker
I don't know the 500, but IIRC, its replacement, the 126, used the tunnel as an air duct, taking air from the engine air-cooling fan to the cabin vents and had a simple, hand-operated damper (just in front of the rear seat) to close it off. If the 500 is similar, does closing that off just before stopping make any difference?

The 500 is the same. I've had the air inlet door closed and it has made no difference. I left the engine cover open over night and that made no difference too. One test I could do is to run the car, then separate the rubber hose from the copper pipe at the back and drain the fuel out of the hose. If it doesn't smell in the morning, the smell is coming from the hose.

coyoteboy - 19/7/23 at 11:01 AM

quote:
Originally posted by rf900rush
Don't forget E10 fuel is not good for older rubber and platics.
I have a 1970's speed boat that had a fibreglass tank which was damaged in weeks of using E10 petrol.

If the fuel lines or carbs use plastic or rubber this may also cause issues.


Suspect this was due to 1970s hoses being at their end of life already. The negative effects highlighted in the govt testing have been largely "tabloided".